/**************************************************************************
 * simpletun.c                                                            *
 *                                                                        *
 * A simplistic, simple-minded, naive tunnelling program using tun/tap    *
 * interfaces and TCP. DO NOT USE THIS PROGRAM FOR SERIOUS PURPOSES.      *
 *                                                                        *
 * You have been warned.                                                  *
 *                                                                        *
 * (C) 2010 Davide Brini.                                                 *
 *                                                                        *
 * DISCLAIMER AND WARNING: this is all work in progress. The code is      *
 * ugly, the algorithms are naive, error checking and input validation    *
 * are very basic, and of course there can be bugs. If that's not enough, *
 * the program has not been thoroughly tested, so it might even fail at   *
 * the few simple things it should be supposed to do right.               *
 * Needless to say, I take no responsibility whatsoever for what the      *
 * program might do. The program has been written mostly for learning     *
 * purposes, and can be used in the hope that is useful, but everything   *
 * is to be taken "as is" and without any kind of warranty, implicit or   *
 * explicit. See the file LICENSE for further details.                    *
 *************************************************************************/

#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/if_tun.h>
#include <net/if.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

/* buffer for reading from tun/tap interface, must be >= 1500 */
#define BUFSIZE 2000
#define CLIENT 0
#define SERVER 1
#define PORT 55555

int debug;
char *progname;

/**************************************************************************
 * tun_alloc: allocates or reconnects to a tun/tap device. The caller     *
 *            must reserve enough space in *dev.                          *
 **************************************************************************/
int tun_alloc(char *dev, int flags) {

    struct ifreq ifr;
    int fd, err;
    char *clonedev = "/dev/net/tun";

    if ((fd = open(clonedev, O_RDWR)) < 0) {
        perror("Opening /dev/net/tun");
        return fd;
    }

    memset(&ifr, 0, sizeof(ifr));

    ifr.ifr_flags = flags;

    if (*dev) {
        strncpy(ifr.ifr_name, dev, IFNAMSIZ);
    }

    if ((err = ioctl(fd, TUNSETIFF, (void *) &ifr)) < 0) {
        perror("ioctl(TUNSETIFF)");
        close(fd);
        return err;
    }

    strcpy(dev, ifr.ifr_name);

    return fd;
}

/**************************************************************************
 * cread: read routine that checks for errors and exits if an error is    *
 *        returned.                                                       *
 **************************************************************************/
int cread(int fd, char *buf, int n) {

    int nread;

    if ((nread = read(fd, buf, n)) < 0) {
        perror("Reading data");
        exit(1);
    }
    return nread;
}

/**************************************************************************
 * cwrite: write routine that checks for errors and exits if an error is  *
 *         returned.                                                      *
 **************************************************************************/
int cwrite(int fd, char *buf, int n) {

    int nwrite;

    if ((nwrite = write(fd, buf, n)) < 0) {
        perror("Writing data");
        exit(1);
    }
    return nwrite;
}

/**************************************************************************
 * read_n: ensures we read exactly n bytes, and puts them into "buf".     *
 *         (unless EOF, of course)                                        *
 **************************************************************************/
int read_n(int fd, char *buf, int n) {

    int nread, left = n;

    while (left > 0) {
        if ((nread = cread(fd, buf, left)) == 0) {
            return 0;
        } else {
            left -= nread;
            buf += nread;
        }
    }
    return n;
}

/**************************************************************************
 * do_debug: prints debugging stuff (doh!)                                *
 **************************************************************************/
void do_debug(char *msg, ...) {

    va_list argp;

    if (debug) {
        va_start(argp, msg);
        vfprintf(stderr, msg, argp);
        va_end(argp);
    }
}

/**************************************************************************
 * my_err: prints custom error messages on stderr.                        *
 **************************************************************************/
void my_err(char *msg, ...) {

    va_list argp;

    va_start(argp, msg);
    vfprintf(stderr, msg, argp);
    va_end(argp);
}

/**************************************************************************
 * usage: prints usage and exits.                                         *
 **************************************************************************/
void usage(void) {
    fprintf(stderr, "Usage:\n");
    fprintf(stderr, "%s -i <ifacename> [-s|-c <serverIP>] [-p <port>] [-u|-a] [-d]\n", progname);
    fprintf(stderr, "%s -h\n", progname);
    fprintf(stderr, "\n");
    fprintf(stderr, "-i <ifacename>: Name of interface to use (mandatory)\n");
    fprintf(stderr, "-s|-c <serverIP>: run in server mode (-s), or specify server address (-c <serverIP>) (mandatory)\n");
    fprintf(stderr, "-p <port>: port to listen on (if run in server mode) or to connect to (in client mode), default 55555\n");
    fprintf(stderr, "-u|-a: use TUN (-u, default) or TAP (-a)\n");
    fprintf(stderr, "-d: outputs debug information while running\n");
    fprintf(stderr, "-h: prints this help text\n");
    exit(1);
}

int main(int argc, char *argv[]) {

    int tap_fd, option;
    int flags = IFF_TUN;
    char if_name[IFNAMSIZ] = "";
    int maxfd;
    uint16_t nread, nwrite, plength;
    char buffer[BUFSIZE];
    struct sockaddr_in local, remote;
    char remote_ip[16] = ""; /* dotted quad IP string */
    unsigned short int port = PORT;
    int sock_fd, net_fd, optval = 1;
    socklen_t remotelen;
    int cliserv = -1; /* must be specified on cmd line */
    unsigned long int tap2net = 0, net2tap = 0;

    progname = argv[0];

    /* Check command line options */
    while ((option = getopt(argc, argv, "i:sc:p:uahd")) > 0) {
        switch (option) {
            case 'd':
                debug = 1;
                break;
            case 'h':
                usage();
                break;
            case 'i':
                strncpy(if_name, optarg, IFNAMSIZ - 1);
                break;
            case 's':
                cliserv = SERVER;
                break;
            case 'c':
                cliserv = CLIENT;
                strncpy(remote_ip, optarg, 15);
                break;
            case 'p':
                port = atoi(optarg);
                break;
            case 'u':
                flags = IFF_TUN;
                break;
            case 'a':
                flags = IFF_TAP;
                break;
            default:
                my_err("Unknown option %c\n", option);
                usage();
        }
    }

    argv += optind;
    argc -= optind;

    if (argc > 0) {
        my_err("Too many options!\n");
        usage();
    }

    if (*if_name == '\0') {
        my_err("Must specify interface name!\n");
        usage();
    } else if (cliserv < 0) {
        my_err("Must specify client or server mode!\n");
        usage();
    } else if ((cliserv == CLIENT) && (*remote_ip == '\0')) {
        my_err("Must specify server address!\n");
        usage();
    }

    /* initialize tun/tap interface */
    if ((tap_fd = tun_alloc(if_name, flags | IFF_NO_PI)) < 0) {
        my_err("Error connecting to tun/tap interface %s!\n", if_name);
        exit(1);
    }

    do_debug("Successfully connected to interface %s\n", if_name);

    if ((sock_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket()");
        exit(1);
    }

    if (cliserv == CLIENT) {
        /* Client, try to connect to server */

        /* assign the destination address */
        memset(&remote, 0, sizeof(remote));
        remote.sin_family = AF_INET;
        remote.sin_addr.s_addr = inet_addr(remote_ip);
        remote.sin_port = htons(port);

        /* connection request */
        if (connect(sock_fd, (struct sockaddr *) &remote, sizeof(remote)) < 0) {
            perror("connect()");
            exit(1);
        }

        net_fd = sock_fd;
        do_debug("CLIENT: Connected to server %s\n", inet_ntoa(remote.sin_addr));

    } else {
        /* Server, wait for connections */

        /* avoid EADDRINUSE error on bind() */
        if (setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, (char *) &optval, sizeof(optval)) < 0) {
            perror("setsockopt()");
            exit(1);
        }

        memset(&local, 0, sizeof(local));
        local.sin_family = AF_INET;
        local.sin_addr.s_addr = htonl(INADDR_ANY);
        local.sin_port = htons(port);
        if (bind(sock_fd, (struct sockaddr *) &local, sizeof(local)) < 0) {
            perror("bind()");
            exit(1);
        }

        if (listen(sock_fd, 5) < 0) {
            perror("listen()");
            exit(1);
        }

        /* wait for connection request */
        remotelen = sizeof(remote);
        memset(&remote, 0, remotelen);
        if ((net_fd = accept(sock_fd, (struct sockaddr *) &remote, &remotelen)) < 0) {
            perror("accept()");
            exit(1);
        }

        do_debug("SERVER: Client connected from %s\n", inet_ntoa(remote.sin_addr));
    }

    /* use select() to handle two descriptors at once */
    maxfd = (tap_fd > net_fd) ? tap_fd : net_fd;

    while (1) {
        int ret;
        fd_set rd_set;

        FD_ZERO(&rd_set);
        FD_SET(tap_fd, &rd_set);
        FD_SET(net_fd, &rd_set);

        ret = select(maxfd + 1, &rd_set, NULL, NULL, NULL);

        if (ret < 0 && errno == EINTR) {
            continue;
        }

        if (ret < 0) {
            perror("select()");
            exit(1);
        }

        if (FD_ISSET(tap_fd, &rd_set)) {
            /* data from tun/tap: just read it and write it to the network */

            nread = cread(tap_fd, buffer, BUFSIZE);

            tap2net++;
            do_debug("TAP2NET %lu: Read %d bytes from the tap interface\n", tap2net, nread);

            /* write length + packet */
            plength = htons(nread);
            nwrite = cwrite(net_fd, (char *) &plength, sizeof(plength));
            nwrite = cwrite(net_fd, buffer, nread);

            do_debug("TAP2NET %lu: Written %d bytes to the network\n", tap2net, nwrite);
        }

        if (FD_ISSET(net_fd, &rd_set)) {
            /* data from the network: read it, and write it to the tun/tap interface.
       * We need to read the length first, and then the packet */

            /* Read length */
            nread = read_n(net_fd, (char *) &plength, sizeof(plength));
            if (nread == 0) {
                /* ctrl-c at the other end */
                break;
            }

            net2tap++;

            /* read packet */
            nread = read_n(net_fd, buffer, ntohs(plength));
            do_debug("NET2TAP %lu: Read %d bytes from the network\n", net2tap, nread);

            /* now buffer[] contains a full packet or frame, write it into the tun/tap interface */
            nwrite = cwrite(tap_fd, buffer, nread);
            do_debug("NET2TAP %lu: Written %d bytes to the tap interface\n", net2tap, nwrite);
        }
    }

    return (0);
}
